Patent Document 1 discloses an exhaust device for an in-line four-cylinder internal combustion engine, in which exhaust ports of cylinders #2 and #3 discontinuous in firing order merge together inside a cylinder head; and exhaust ports of cylinders #1 and #1 are respectively open at a side surface of the cylinder head. Namely, the exhaust ports of cylinders #2 and #3 are configured as one collective exhaust port; and the exhaust ports of cylinders #1 and #4 are configured as respective separate individual exhaust ports. The collective exhaust port of cylinders #2 and #3 are connected to a catalytic converter through one collective exhaust pipe. The individual exhaust ports of cylinders #1 and #4 are connected to the catalytic converter through respective separate individual exhaust pipes.
The exhaust device in which the exhaust ports of some cylinders merge together inside the cylinder head is advantageous for early catalyst activation after engine start-up because the temperature of exhaust gas introduced to the catalytic converter through the collective exhaust pipe can be maintained at a high level during cold engine start-up. Further, it is described in Patent Document 1 that the length of the collective exhaust pipe for cylinders #2 and #3 is set shorter than that of the individual exhaust pipes for cylinders #1 and #2 so as to suppress heat radiation from the collective exhaust pipe.
In the exhaust device in which the exhaust ports of some cylinders merge together inside the cylinder head, however, the temperature of exhaust gas introduced to the catalytic converter through the collective exhaust pipe tends to become too high during high-speed high-load engine operation after engine warm-up. This can lead to catalyst deterioration even though the exhaust device is advantageous for early catalyst activation after engine start-up as mentioned above.
Namely, there is a demand to introduce the exhaust gas to the catalytic converter, while maintaining the temperature of the exhaust gas as high as possible, during cold engine start-up for the purpose of early catalyst activation. On the other hand, there is also a demand to suppress the temperature of the exhaust gas introduced to the catalytic converter during high-speed high-load engine operation. It is difficult for the conventional exhaust device to satisfy both of these mutually contradictory demands.